One of the promising methods for data writing in MRAMs proposed in recent years is the spin momentum transfer, which involves injecting a spin-polarized current as a write current into a magnetic recording layer and consequently inverting the magnetization of the magnetic recording layer. The spin momentum transfer allows decreasing the necessary current with the reduction of the size of the memory cell, while the magnetization inversion by using the current magnetic field is accompanied by an increase in the necessary current with the decrease in the size of a memory cell. Thus, the spin momentum transfer is considered as a promising method to realize an MRAM with an increased capacity.
However, the use of spin momentum transfer for a magnetic tunnel junction device requires overcoming the problem of the tunnel barrier layer breakdown. In the current technique, the spin-polarized current of several mA or more is required to be injected into the magnetic recording layer, when spin momentum transfer is used to invert the magnetization. However, feeding such a large current through the magnetic tunnel junction may result in the breakdown of the tunnel barrier layer.
One approach for overcoming the problem of the tunnel barrier layer breakdown is a technique that feeds a spin-polarized current in the in-plane direction of the magnetic recording layer to cause magnetization inversion. Such techniques are disclosed in, for example, Japanese Laid Open Patent Application (P2005-191032A), Japanese Laid Open Patent Application (P2005-123617A), and U.S. Pat. No. 6,781,871. Feeding a spin-polarized current in the in-plane direction of the magnetic recording layer allows moving the magnetic wall of the magnetic recording layer and/or exerting a torque on the magnetization of the magnetic recording layer by the spin-polarized current, consequently inverting the magnetization of the magnetic recording layer. The technique which feeds a spin-polarized current in the in-plane direction of the magnetic recording layer eliminates the need for flowing the spin-polarized current through the tunnel barrier layer, effectively avoiding the problem of the breakdown of the tunnel barrier layer.
According to a study of the inventor, one issue of the MRAM in which a spin-polarized current is flown in the in-plane direction of the magnetic recording layer is the heat generation in the magnetic recording layer. The resistance of the magnetic recording layer is inevitably high in the in-plane direction, since ferromagnetic material has a high electric resistivity in general. On the other hand, the inversion of the magnetization of the magnetic recording layer requires a high spin-polarized current to some extent. Therefore, feeding the spin-polarized current in the in-plane direction of the magnetic recording layer for data write increases the temperature of the memory cell. The temperature increase in the memory cell undesirably causes the drop in the reliability of the MRAM operation. It is desired to provide a technique for suppressing the temperature increase caused by the write current flowing in the in-plane direction of the magnetic recording layer.